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De Jong, an Auckland University of Technology student with an infectious smile that could "cheer anyone up", was swimming with friends in the river when the Aratiatia Dam floodgates opened.

Four of the group were swept away and only three made it back to shore.

De Jong's body was later recovered from a rockpool.

Her brother Daniel De Jong last night posted a moving tribute to his beloved sister on social media.

"Yesterday I lost one of the most important people in my life, my wonderful sister. Not only was she an inspiration to us all, she was my best friend, and the most perfect sister I could've ever asked for.

"I can't even begin to describe how much I'm going to miss you, and how incredibly unfair it is that you have been taken far, far too soon.

The dam's gates open four times a day in summer so tourists can watch the rapids from a viewing platform. The water is calm when the gates are closed but quickly rises when they open.

The section of the river near the dam is fenced off to prevent people swimming.

Signs alerting people to the danger are also in place on the bank and sirens are let off before the gates open.

The town's mayor, David Trewavas, told the Herald he had spoken to the dam's operator Mercury Energy, which is reviewing its safety procedures after the drowning, and it appeared not much more could have been done to prevent the death.

"It's an absolute tragedy what's happened but it sounds like the systems were in place and the signs were up and the siren went off, so it's one of those unfortunate things."

Mercury Energy chief executive Fraser Whineray confirmed yesterday the sirens were working when the drowning happened.

The death follows a close call in the same spot in 2009 when two women were swept down the river after the floodgates opened and they had to be plucked to safety by kayakers.